BiologyMrs. DoumaEvolution

The Chromosome Connection

Activity

SELF-CHECK:

Based on your reading of the Background Information, check your understanding now by answering the following questions on the worksheet provided.

1.  If two different species have extensive identical chromosome banding patterns, they...

a.  Must be related

b.  Are probably related

c.  Could be related, but not necessarily so

d.  Are probably not related

e.  May or may not be related…no basis for judging this

2.  If you found that two sets of chromosomes (one from species "X" and one from species "Y") showed a very close match (in the number of chromosomes and many identical banding patterns), it would be most reasonable to say that...

a.  One species evolved from the other

b.  Both species evolved from a recent common ancestor

c.  Both species may be distantly related

d.  Their chromosome similarities are only a striking coincidence

PART 1: Matching Bullet Marks

Imagine yourself as a scientist in training to become a CSI (Crime Scene Investigator). One of your tasks is to practice finding a match for bullets fired from the same gun. We know that bullets fired from the same gun have very similar markings caused by spiral grooves and imperfections inside the barrel of the gun.

Six guns were involved in a recent crime. Gun specialists fired each gun into a test chamber and retrieved the spent bullet from each gun. The bullet marks were photographed. You are shown photos of the scratch marks on those 6 bullets. You are then given a photo of the scratch marks on bullet (B) taken from that crime scene. Your job is to figure out which gun the crime scene bullet (B) was fired from.

1.  Remove from your Chromosome Connection Envelope the strip of paper for Part 1, showing the bullet scratch pattern of bullet B, and the group of 6 other bullet scratch patterns (#1-6).

2.  Place the B pattern next to each of the 6 patterns in turn. Which pattern matches the B pattern?

Return these two paper strips to the envelope when done.

Since there is a perfect match, this tells you that these two bullets DID come from the SAME GUN...they both have a common origin.

PART 2: Identical Chromosomes?

Identical scratch patterns on bullets indicate they came from the same gun. The same principle applies when we look at chromosomes, the units of heredity found in each cell.

Identical chromosomes have a common origin. They are inherited from the same source: a common ancestor.

Chromosomes, when treated with a particular stain, reveal characteristic banding patterns, according to their molecular makeup. Where there are many cytosine-guanine pairs in their DNA, they stain very darkly.

On the right are the detailed diagrams of chromosomes from three different animals which share many characteristics with humans. In fact, even these chromosomes are very similar to the human chromosome #3.

Look at the handout that compares the karyotype of two species. Can you find any chromosomes that are a perfect match?

That’s it!!! These two chromosomes match perfectly, band for band. Just as with the perfectly matching bullet scratch marks, this is taken as clear evidence that they must have had a common origin. Even though these chromosomes come from two different species, they both had to have a common ancestor. The perfectly matching chromosome is from a chimpanzee, so this tells us that humans and chimps must have both descended from a fairly recent common ancestor that was neither human nor chimp.

PART 3: Alike, but Different: Inversions

Sometimes similar chromosomes don’t match perfectly. This is because even if humans and chimps once shared a common ancestor, they have both changed since that time, and it’s reasonable to expect that their chromosomes have changed also. But even apparent differences can be more superficial than real.

Cut out the chromosomes and see if you can make them match.

You’ve just produced an INVERSION! (This is a “pericentric” inversion because it’s around the centromere). Analysis suggests that this is exactly how the chimp chromosome was formed from the chimp/human ancestor. In fact, according to the authors of that study (Yunis and Prakash, 1982), the human chromosome #4 is considered as the ancestral type “because it’s the only one from which the others [chimp, gorilla, and orangutan] can be derived by a single, but differing, pericentric inversion ...” That ancestral chromosome #4 has continued unchanged in humans.

Scientists typically infer the simplest explanation that satisfies all the evidence, unless there are compelling reasons to do otherwise. This is called “parsimony”

The tell-tale signs of inversion have been observed in many organisms. It is very common and normal, and typically occurs during meiosis. During this process, chromosomes are seen in peculiar configurations called “chiasmata” (crosses), as shown here. Chromatids twist about each other, sometimes forming loops. When these break and reattach (crossing over), segments may be reversed (inverted).

Many of the chromosomes in apes and humans are essentially identical except for their inversions. We know from numerous examples that inversions can occur (in humans and other animals) without affecting their fertility and normal development.

PART 4: Alike, but Different: Fission or Fusion?

In addition to inversions, chromosomes undergo other changes which can account for some of the differences seen in ape and human chromosomes. Sometimes, a chromosome will break apart, producing two shorter chromosomes from one longer one. This is called “fission”. Other times, two short chromosomes will stick together, end to end, forming a single long chromosome where there were two before. This is called “fusion”

Humans have 23 pairs of chromosomes, chimps and the other apes have 24. Could this have happened as a result of an early ancestor chromosome splitting apart to produce the two smaller chromosomes (fission) we find in apes? Or, was there a common ancestor in which two smaller chromosomes linked together to make one, but only in the human line of descent? Let’s see if we can find out. When we study the chromosomes carefully, we find that the long human chromosome #2 (shown at right) has banding patterns that look very similar to two shorter chromosomes found in apes (which we’ll call “2p” and 2q”).

In your Envelope, find the two “Part 4” strips, one marked “2p” and the other marked “2q”. These are chimpanzee chromosomes. Now place them end to end, and align them next to the #2 human chromosome on the right. If they don’t match, invert one or the other chromosomes, and/or exchange the “attachment” ends, until you get the combination that matches the human #2 chromosome. How would you describe that match comparison?

A) Identical

B) Very similar

C) Somewhat similar

D) Totally different.

Detailed studies have shown that human chromosome #2 was most likely the result of the fusion of the two smaller chromosomes found in the common ancestor of chimps and humans (and still found in chimps). According to Yunis and Prakash (1982), the “... ancestral chromosome 2p [was probably] similar to that of orangutan and gorilla, with a pericentric inversion accounting for the chimpanzee 2p. The ancestral 2q, on the other hand, resembled that of gorilla and chimpanzee, and [the] human chromosome #2 can be explained [most simply] by fusion of a chimpanzee-like 2p and the ancestral 2q.” This, in fact, is one of several indications that humans and chimps are more closely related to each other than either is to gorillas, and that orangutans are even more distantly related.

PART 5: How do the Other Apes Compare?

Not only do humans and chimps have very similar chromosomes, but (as you may have suspected by now) their chromosomes are also very similar to those of gorillas and orangutans.

From the Chromosome Connection Envelope, remove the paper showing 7 selected sets of chromosomes, with 4 chromosomes in each set. Each set consists of the corresponding chromosome from each of four species. In some cases, you may find that one chromosome in a set is significantly different from the other three. In other cases, all four are very similar, with none significantly different.

As you compare the chromosomes in each set, you can ignore the dark-staining tip-ends found on some chromosomes...they are composed of “heterochromatin”, and are not considered as important differences. For each item below, if none of the sets show the specified difference, say “NONE of these”

1.  Look for those sets in which the fourth (O) chromosome is rather different from the other three. You might want to discuss your selections with a partner. Record the numbers of those sets.

2.  Look for those sets set in which the third (G) chromosome is clearly different from the other three. Record the number (or numbers) selected.

3.  Look for those sets in which the 1st or 2nd chromosome (H or C) is rather different from the others. Record the number (or numbers) selected.

CHECK QUESTIONS

3.  How would you now compare the chromosomes (generally) of these four organisms (H, C, G O)?

a.  Identical

b.  very similar

c.  somewhat similar

d.  mostly different

e.  totally different

4.  Based on the above observation (and the background information), what would be the most logical assumption about the relationship between these four organisms?

f.  they are members of the same species

g.  they all evolved from a common ancestor relatively recently

h.  they are most likely distantly related

i.  no basis for an assumed relationship...could be related or totally unrelated

NEW INFORMATION: Species C, G, and O are all in different genera which, based on anatomy, have all been traditionally classified in the same taxonomic family: the pongids (great apes), while humans have been placed in the hominid family. However, when we compare the full sets of chromosomes for all 4 species, we find that

1.  All four species are very similar

2.  The chimp set is more like the human set than either is like the other apes.

CHECK QUESTION

5.  Given this information, should apes and humans all be placed in the same classification family? Yes/No Why?

Questions

Write the answers to these questions in your notebook in a narrative:

1.  Regarding the hypothesis that humans and chimpanzees share a common ancestor, how much did your acceptance change from the first to the last piece of evidence you studied? How much did your confidence in your conclusions change as you accumulated more evidence?

2.  Are you 100 percent sure that the hypothesis that humans and chimpanzees share a common ancestor is true? Explain.

3.  What data did you find the least convincing? What data did you find the most convincing? Explain your choices.

4.  Could all the data be recreated by another scientist? Why is this important in science?